SAP AND SECRETIONS. 199 



\y called the sap. It is really the blood of the plant, by 

 which its whole body is nourished, and from which the pecu- 

 liar secretions are made. 



The great motion, called the flowing of the sap, which is 

 to be detected principally in the spring, and slightly in the 

 autumn, is totally different from that constant propulsion of 

 it which is going on in every growing plant. Its facility to 

 run is the first step towards the revival of vegetation from 

 the torpor of winter. Its exciting cause is heat, and the ef- 

 fect of heat is in proportion to the degree of cold to which the 

 plant has been accustomed. The same principle accounts 

 for the occasional flowing of the sap in autumn after a slight 

 frost. Such a premature cold increases the sensibility of 

 the plant to any warmth that may follow, and produces, in a 

 degree, the same state of its constitution as exists after the 

 long and severer cold of winter. 



The sap in its passage through the leaves and bark be- 

 comes quite a new fluid, possessing the peculiar flavour and 

 qualities of the plant, and not only yielding woody matter 

 for the increase of the vegetable body, but furnishing various 

 secreted substances. These are chiefly found in the bark, 

 and often in large and conspicuous vessels, as the turpen- 

 tine-cells of the Fir tribe. In herbaceous plants, whose 

 stems are only of annual duration, the perennial roots fre* 

 quently contain these fluids in the most perfect state, nor 

 are they, in such, confined to the bark, but deposited through- 

 out the substance of the root, as in Rhubarb and Gentian. 

 It may be useful to enumerate some of the most distinct se- 

 cretions of vegetables. Gum or mucilage, a viscid sub- 

 stance of little flai'our, exudes from many trees in the form 

 of large drops or lumps, as in Plum, Cherry, and Peach trees. 

 Resin is a substance soluble in spirits, and it differs ac- 

 cording to the peculiar tree from which it is obtained. The 

 more refined and volatile secretions of a resinous nature are 

 called essential oils, and they are often highly aromatic and 

 odoriferous. They exist in the highest perfection in the 

 perfumed effluvia of flowers, some of which, capable of 

 combination with spirituous fluids, are obtainable by dis- 

 tillation, as that of the Lavender and Rose. The bitter 

 secretion of many plants does not seem exactly to accord 

 with any of the foregoing. Some facts would seem to 

 prove it of a i^sinous nature, but it is often perfectly 



